1. Field of the Invention
The present invention relates to a vehicle integrated control apparatus which suppresses a phenomenon of uncontrollable steering caused by a steering torque change during a vehicle traveling in situations where the vehicle attitude becomes unstable, such as the case where a right/left-wheel differential limiting mechanism operates on a rough road in which the road surface friction coefficients of the right and left front wheels are different from each other, thereby improving the traveling performance and stability of a vehicle.
2. Description of the Related Art
In the field of a power steering system of a vehicle, recently, as an aid to the steering torque caused by a steering operation by the driver, an electric power steering system has been employed that is a steering assisting mechanism which can control a steering force by adding a steering assisting force imparted by the force of an electric motor, to the steering operation.
In such an electric power steering system, a friction is larger because of the resistance of the electric motor and the like as compared with a hydraulic power steering system or the like, and the steering is hardly returned to a neutral position. Therefore, a technique has been proposed in which, during a steering operation, an electric power steering system is controlled by using the right/left-wheel speed difference between the steered wheels so that a steering assisting force is added in the direction of returning the steering to the neutral position.
In a vehicle which includes a right/left-wheel differential limiting mechanism in order to improve the traveling performance of the vehicle, in the case where the right/left-wheel differential limiting mechanism operates during turning traveling, traveling on a μ-split road in which right and left tires receive different road surface frictions, or the like, uncontrollable steering occurs, and there is a fear that the driver might have a feeling of strangeness during a steering operation.
As a steering force control mechanism in a vehicle using such a right/left-wheel differential limiting mechanism, proposed is an apparatus in which an electric power steering system functioning as a steering assisting mechanism is controlled in accordance with a restraining torque produced when the right/left-wheel differential limiting mechanism operates, and the occurrence of the phenomenon of uncontrollable steering caused by a steering torque change is suppressed so that the driver's feeling of strangeness during a steering operation is eliminated (see Japanese Patent No. 3,401,336).
However, an actual steering torque change is proportional not to the restraining torque of a right/left-wheel differential limiting mechanism, but to the driving force difference between the right and left wheels. When a control which is proportional to the restraining torque of a right/left-wheel differential limiting mechanism is performed as in the above-described prior art, therefore, the control amount becomes excessive or insufficient. In a state where the right/left-wheel differential limiting mechanism is locked, i.e., where the right and left wheels are directly coupled with each other, the direction and degree of a steering torque change are not known, and, for example, it is difficult to perform a control based on an electric power steering apparatus which is a steering assist apparatus. Therefore, there is a problem in that the proposed apparatus is insufficient to eliminate the driver's feeling of strangeness during a steering operation.
In a usual mechanical right/left-wheel differential limiting mechanism, moreover, it is difficult to estimate a steering torque change. At the same time, a steering torque change is hardly suppressed by an electric power steering system.
On the other hand, when assuming an electronic right/left-wheel differential limiting mechanism which can freely control the restraining torque of the right/left-wheel differential limiting mechanism, a steering torque change can be accurately estimated by applying a control logic for the right/left-wheel differential limiting mechanism which can easily estimate a steering torque change due to the right/left-wheel differential limiting mechanism.